Atomic force microscope

ABSTRACT

This atomic force microscope has a probe for surface analysis of a sample (E), comprising a support body and an elastically deformable strip linked to the body, the strip being provided with a tip designed to come into contact with the sample (E) to be analysed. The microscope also has a mechanism for relative displacement of the analysis probe with respect to the surface of the sample (E), a detector for determining the position of the strip, and elements for vibrating the strip. These means for vibrating the strip include elements for conduction of electricity along a continuous path forming a loop, an alternating-current generator, and a magnetic-field source designed to set up a magnetic field ({right arrow over (B)}) in the region of the strip of the analysis probe.

The present invention relates to an atomic force microscope, of the typehaving a probe for surface analysis of a sample, comprising a supportbody and an elastically deformable strip linked to the body, the stripbeing provided with a tip designed to come into contact with the sampleto be analysed; a mechanism for relative displacement of the analysisprobe with respect to the surface of the sample; a detector fordetermining the position of the strip; and means for vibrating thestrip.

In an atomic force microscope, the probe for analysis of the surface andthe sample are moved relative to one another along lines.

Such a probe has an elastically deformable strip carried at the end of asupport body. This strip is commonly referred to by the English term“cantilever”. At its free end, the strip has a tip designed to enterinto contact with the surface of the sample to be analysed. The bendingof the strip is measured, in particular by optical means, in order todetermine the effects of the mechanical interaction between the surfacebeing studied and the tip disposed at the end of the strip.

It is known, in order to reduce damage to the surface of the sample tobe analysed, to vibrate the strip in a direction perpendicular to theplane of the sample, so that the tip taps the surface of the sample. Theinteractions of the tip with the surface generate changes in theamplitude response or phase response of the strip, and these provideaccess to the surface properties of the sample, such as topography orelasticity. This analysis mode is generally referred to by the term ACmode.

In order to vibrate the strip, it is known, in particular from thedocument WO-99/06793, to add a particle or a deposit of a magneticmaterial on the top of the strip, and to create an oscillating magneticfield in the space where the strip is displaced. This magnetic field iscreated, for example, by a coil through which an alternating currentflows. This way of vibrating the strip is advantageous in comparisonwith conventional methods in which the vibration is inducedmechanically, because it eliminates the resonances due to thismechanical system. This is particularly borne out for use in an aqueousmedium.

The homogeneous deposition of a magnetic material on the strip is adifficult operation. Furthermore, this material changes the mechanicalcharacteristics of the strip.

It is an object of the invention to provide an atomic force microscope,and a surface-analysis probe for this microscope, which can operatesatisfactorily in a mode in which the strip is vibrated and in which thestrip can be fabricated easily.

To that end, the invention relates to a microscope of the type alreadymentioned, characterized in that the means for vibrating the strip have,on the strip, means for conduction of electricity along a continuouspath forming a loop, which electrical-conduction means are secured tothe strip, the support body being provided with two divided conductivesections extending the loop, an alternating-current generator connectedto the divided conductive sections of the analysis probe, and amagnetic-field source designed to set up a substantially homogeneousmagnetic field in the region of the strip of the analysis probe.

The invention also relates to a microscope of the type alreadymentioned, characterized in that the detector has, on the strip, meansfor conduction of electricity along a continuous path forming a loop,which electrical-conduction means are secured to the strip, amagnetic-field source designed to set up a substantially homogeneousmagnetic field in the region of the strip of the analysis probe, andmeans for analysis of currents induced by the magnetic field in theloop.

One or other of these microscopes according to the invention may have,in any technically feasible combination, one or more of thecharacteristics described in claims 2 and 4 to 12.

The invention will be, understood more clearly on reading the followingdescription, which is provided by way of example and is given withreference to the drawings, in which:

FIG. 1 is a diagrammatic view of the surface-analysis probe arranged ina microscope according to the invention;

FIG. 2 is a view showing the change in the amplitude of the vibrationsof the elastic strip of an exemplary embodiment of an analysis probe inthe presence of an excitation current whose frequency varies from 0 to40 kHz; and

FIG. 3 is an image of a sample obtained with a microscope using a probeaccording to the invention; and

FIGS. 4 and 5 are views similar to the one in FIG. 1, each illustratingan embodiment of a variant of a microscope according to the invention.

The atomic force microscope 10, only the main elements of which arerepresented in FIG. 1, has a probe 12 for analysis of the surface of asample E. The microscope furthermore has a movable table 14 forsupporting the sample. This table 14 can be moved, in three orthogonaldirections, relative to the analysis probe 12 under the action ofdisplacement means 16 of any suitable type. These means produce relativedisplacement of the probe 12 with respect to the sample E substantiallyin the plane of the surface of the sample E. They are designed to makeit possible to scan the surface of the sample with the probe 12.

The microscope 10 furthermore has a permanent magnet 18 which ispreferably arranged under the movable table 14. This permanent magnet 18is capable of creating a permanent magnetic field {right arrow over (B)}in the region of observation of the sample. The magnet is selected sothat the field {right arrow over (B)} is set up substantiallyhomogeneously in the region where the analysis, end of the probe isdisplaced. This field {right arrow over (B)} is substantiallyperpendicular to the surface of the sample, i.e. to the direction inwhich the sample is scanned by the probe.

The analysis probe 12 has a support body 20 which permits mechanicallinkage of the probe to the structure of the atomic force microscopeand, more precisely, to the displacement means 16 integrated therein.

The support body 20 has a parallelepipedal overall shape and isessentially made of glass or “pyrex”. One side face of the support body20 is extended by an elastically deformable strip 22. This strip isobtained by subsequent deposition on the body of silicon nitride Si₃N₄.The strip 22 has a small thickness which is very much less than that ofthe body, hence permitting its elastic deformation. Its thickness is,for example, 1 μm. The length of the strip is, for example, 200 μm.

The strip 22 is essentially flat and extends parallel to the surface ofthe body 0.20. The strip is secured to the body 20 only at one end. Itis hence cantilevered.

The analysis probe is carried in the microscope by a support whichensures that the strip 22 is slightly inclined relative to the surfaceof the sample E. The normal to the strip 22 is, for example, inclined byabout 10° relative to the normal to the overall plane of the sample E.The normal to the plane 22 hence defines an angle of about 10° with themagnetic field {right arrow over (B)}.

At its free end, the strip 22 carries a tip 24 designed to enter intocontact, in an aqueous medium, with the sample E to be analysed. The tipextends perpendicularly to the plane of the strip 22. The radius ofcurvature of the tip 24 is about 30 nm at its extreme end.

The strip 22 advantageously has two coplanar branches 26A, 26B whichconverge towards each other. These two branches have distinct roots,where they are linked to the support body 20. Their other ends arelinked to each other by a junction bend, where the tip 24 is carried.

According to the invention, the strip 22 is provided with means forconduction of electricity along a continuous path forming a loop 27which extends over the essential part of the length of the strip. Thisloop is secured to the said strip.

In the embodiment which is represented, these electrical-conductionmeans are formed by a conductive coating which covers at least one ofthe faces of the strip, so as to create a conductive path. This coatingis advantageously a metal coating, preferably consisting of gold. Itsthickness is between 5 nm and 500 nm and is advantageously substantiallyequal to 50 nm.

The loop 27 formed by the metal coating extends along the two branches26A, 26B. It is therefore formed by a single turn which is open in theregion of the support body 20.

This loop is extended by divided conductive sections 30A, 30B disposedon the surface of the support body 20. These conductive sections areformed by a conductive coating extending over the main surface of thebody, and over its side surface to which the strip 22 is connected.Advantageously, the conductive coating extending over the support bodyis of the same nature as the coating applied-to the strip. They arepreferably formed simultaneously and constitute a single coatingextending both over the strip and over the support body.

The sections 30A and 30B are separated from each other by an insulatingarea 32 free of conductive coating.

According to the invention, an alternating electric-current generator 34is connected between the two conductive areas 30A and 30B. To that end,conductive pads forming the terminals of the generator 34 are heldapplied to the conductive sections 30A and 30B, the latter henceforming, in the region where the conductive pads are applied, means forconnection of the loop 27 to the alternating-current generator 34.

The electric current which is delivered by the generator and flowsthrough the loop 27 is preferably sinusoidal and its strength is between0.01 and 100 mA.

Lastly, the atomic force microscope has a light source 50 whichilluminates the surface carrying the metal coating of the strip 22, aswell as a receptor 52 for detecting the position of the reflected beam.The emitter 50 and the receptor 52 are designed, as is known per se, todetermine the change in the deflection of the elastic strip 22.

The atomic force microscope operates in the following way.

During the analysis of the surface of a sample E, thealternating-current generator 34 is turned on, hence causing anoscillating electric current to flow through the loop 27 formed by theconductive coating carried by the strip 22.

The flow of the alternating electric current through this loop creates avariable magnetic moment {right arrow over (M)}. At each instant, thepresence of the magnetic field {right arrow over (B)} generated by thepermanent magnet 18 imposes stresses on the strip 22, the magneticmoment {right arrow over (M)} thereof tending to align with the magneticfield {right arrow over (B)}. In other words, the current loop formed onthe strip is subjected to Laplace forces which are transmitted to thestrip 22, the latter being mechanically secured to the loop. Thestresses applied to the strip result in continual deformation of thelatter, as a function of the change in the magnetic moment {right arrowover (M)}.

Because of the periodic reversal of the direction of the current set upin the loop by the alternating-current generator 34, the magnetic momentreverses at each half-period of the generator, hence creatingoscillation of the strip 22 and, in particular, of its end carrying thetip 24.

Advantageously, the frequency of the alternating-current generator 34 isset to the resonant frequency of the strip 22, or close to thisfrequency.

FIG. 2 represents the vibration amplitude of the strip 22 immersed inwater, for a current of the order of 1 mA flowing through the loop 27,as a function of frequency.

In this figure, the frequency in hertz is given on the abscissa, whereasthe amplitude of the oscillations in nanometres is represented on theordinate.

The sinusoidal current flowing through the turn is frequency-modulatedbetween 0 and 40 kHz and its peak strength is about 1 mA. Thewell-defined amplitude maximum observed at about 4 kHz corresponds tothe natural vibration frequency of the strip, i.e. to its resonantfrequency.

The strip which is used has a stiffness constant of the order of 50mN/m. The current loop was produced by depositing a 50 nm gold film onthe lower face of the strip, i.e. the face from which the tip emerges.

The very large amplitude observed for the vibrations is commensuratelyhigher if the frequency of the generator is close to the naturalresonant frequency of the strip.

FIG. 3 represents an image of the topography of actin filamentsdeposited on the surface of a glass slide, the image having been takenwith the proposed probe by using a suitable atomic force microscope.Each grey level of this image corresponds to an “altitude” on thesample, as indicated by the scale.

This image is taken in water using the AC mode.

The excitation of the resilient strip is generated by virtue of asinusoidal current flowing through the loop carried by the strip, with apeak strength of the order of 1 mA. Its frequency is set at 3.7 kHz(close to the natural frequency). The output signal obtained on thephotodetector is about 5 Vrms, corresponding to a vibration amplitude ofabout 40 nm.

With a device as described here, it will be understood that thevibration of the strip in an atomic force microscope is simplified sinceit only requires the installation of a permanent magnetic-field sourceand an alternating-current generator. Furthermore, the placement of aconductive coating on the surface of the strip 22 is a well-masteredtechnique which is commonly used in the semiconductor industry, wherethe deposition of a metal coating is a standard operation.

In an alternative embodiment of the microscope, the position of themagnet 18 creating the magnetic field {right arrow over (B)} ismodified. In particular, the magnet is not placed with its axissubstantially perpendicular to the surface of the sample to be analysed.Instead, the magnet is disposed in such a way as to produce a magneticfield whose direction extends parallel to the surface of the sample tobe analysed, specifically in a direction parallel to the longitudinalmid-axis of the strip.

Under these conditions, when a sinusoidal current flows through the loop27 carried by the strip, the strip 22 experiences twisting about itslongitudinal mid-axis. Hence, under the action of the reciprocatingtorsional movement of the strip, the tip 24 executes reciprocating localscanning of the surface of the sample, hence making it possible tomeasure the frictional properties of the surface of the sample.

These frictional properties are deduced from analysis of the movement ofthe light beam reflected by the strip.

In a variant of the microscope according to the invention, the sample Eto be analysed is fixed and the analysis probe 12 can be moved relativeto this sample under the action of displacement means of any appropriatetype. The operation of the microscope is then substantially similar tothat of the microscope in FIG. 1.

According to an alternative embodiment, which is not shown, a pluralityof strips which are substantially similar to one another are organizedon the same probe. Each strip is provided with its ownelectrical-conduction loop, each loop being supplied in parallel by thealternating-current generator. Advantageously, the microscope has meansfor generating a substantially homogeneous magnetic field {right arrowover (B)} in the region where the analysis ends of these strips aredisplaced. These means are formed, for example, by a correspondinglydimensioned permanent magnet. During the displacement of the probe,since an alternating current flows through each strip, it will beunderstood that each strip vibrates independently. During the samescanning pass by the probe, the bending of each of the strips ismeasured, which makes it possible to scan the surface to be analysedmore quickly.

Furthermore, according to yet another variant, the microscope does nothave an external strip-position detector such as the optical system withlight beams 50, 52 in FIG. 1.

According to a first embodiment of this variant, which is represented inFIG. 4, the loop 27 is not connected to an alternating-currentgenerator. However, a component 60 for mechanical excitation of thestrip is applied to the probe. This mechanical-excitation component iscomposed of, for example, an auxiliary piezoelectric mechanism whichvibrates the strip 22 carrying the loop.

The loop 27 is connected to means 70 for processing the signal due tothe induced currents. In particular, these means have amplificationmeans 72 associated with means 74 for measurement of the voltage and/orthe strength of the current in the loop 27, as well as means 76 foranalysis of the amplified signals. Advantageously, the magnetic field{right arrow over (B)} produced by the source 18 is very high, beingmore than 0.2 T and, for example, equal to 1 T.

During the relative displacement of the probe 12 above the sample E tobe analysed, since the strip 22 of the probe is mechanically excited,the oscillation of the loop 27 in the magnetic field {right arrow over(B)} gives rise, in the loop, to an electromotive force which generatesinduced currents. This electromotive force is analysed by thesignal-processing means 70 connected to the terminals of the loop. Theanalysis of the signals amplified by the aforementioned means 76 makesit possible to determine the profile of the surface under analysis, theelectromotive force being representative of the interaction by contactbetween the measurement end of the probe and the sample surface to beanalysed.

According to another embodiment, which is represented in FIG. 5, thepiezoelectric excitation mechanism is absent and, in addition to thesignal-processing means 70, the loop 27 is connected to analternating-current generator 34 as in the embodiment in FIG. 1.

During the analysis, the generator 34 causes an alternating current toflow through the loop 27. Under the action of the flow of this current,the strip 22 is made to vibrate, and to enter into contact with thesurface to be analysed during the relative displacement of the probe 12.

Because of the oscillatory movement of the loop 27 in the magnetic field{right arrow over (B)}, a back electromotive force is formed in the loopand generates induced currents therein. This back electromotive force isanalysed by the signal-processing means 70, in order to deduce therefromthe profile of the surface of the analysed sample.

In practice, it will be understood that the electromotive force or theback electromotive force produced in the loop 27 has a voltage ofbetween 0.1 nV and 10 nV, corresponding to a strength of between 1 pAand 10 nA, for example, a voltage of about 1 nV corresponding to astrength of about 100 pA.

With such means for detection of the deformation of the strip 22, it ishence possible to determine the profile of the surface to be analysedwithout employing optical means.

1. Atomic force microscope, comprising a probe (12) for surface analysisof a sample (E), comprising a support body (20) and an elasticallydeformable strip (22) linked to the body (20), the strip being providedwith a tip (24) designed to come into contact with the sample (E) to beanalysed; a mechanism (16) for relative displacement of the analysisprobe (12) with respect to the surface of the sample (E); a detector(50, 52) for determining the position of the strip (22); and means forvibrating the strip (22), said means for vibrating the strip comprising:on the strip (22), means (26A, 26B) for conduction of electricity alonga continuous path forming a loop (27), which electrical-conduction means(26A, 26B) are secured to the strip (22), the support body (20) beingprovided with two divided conductive sections (30A, 30B) extending theloop (27), an alternating-current generator (34) connected to thedivided conductive sections (30A, 302) of the analysis probe (12), and amagnetic-field source (18) designed to set up a substantiallyhomogeneous magnetic field ({circle around (B)}) in the region of thestrip (22) of the analysis probe (12), wherein the loop (27) has asingle turn which is open in the region of the support body (20). 2.Microscope according to claim 1, wherein the detector has means (76) foranalysis of currents induced by the magnetic field ({circle around (B)})in the loop (27).
 3. Microscope according to claim 1, wherein the strip(22) has two branches (26A, 26B) which converge towards each other fromdivided roots for linkage to the support body (20) as far as a junctionbend which carries the tip (24), and wherein the continuous path formingthe loop (27) extends along the length of the two branches (26A, 26B).4. Microscope according to claim 1, wherein the electrical-conductionmeans have a conductive coating which is formed on one face of the strip(22) and extends over the support body (20) to form the two dividedconductive sections (30A, 302), the latter being separated by acoating-free area (32) of the body.
 5. Microscope according to claim 1,wherein the magnetic-field source (18) is designed to set up a permanentmagnetic field ({circle around (B)}) in the region of the strip (22). 6.Microscope according to claim 1, wherein the alternating-currentgenerator (34) has means for controlling the frequency of the current,which are designed to bring the latter to a frequency substantiallyequal to the resonant frequency of the strip (22).
 7. Microscopeaccording to claim 1, wherein the magnetic-field source (18) is arrangedto set up a magnetic field ({circle around (B)}) whose direction extendssubstantially transversely to the plane of the strip (22).
 8. Microscopeaccording to claim 1, wherein the magnetic-field source is arranged toset up a magnetic field whose direction extends substantially along thelongitudinal mid-axis of the strip.
 9. Microscope according to claim 1,wherein, in use, the means for vibrating the strip (22) create amechanical interaction between the surface of the sample (E) and the tip(24) of the strip (22).
 10. Microscope according to claim 1, wherein theradius of curvature of the tip (24) is about 30 nm at its extreme end.11. Microscope according to claim 1, wherein, in use, the flow of analternating electric current from the generator (34) through the loop(27) creates a variable magnetic moment ({circle around (M)}) whichleads to align with the magnetic field ({circle around (B)}) set up bythe magnetic field source (18) and which consequently deforms the strip(22) in a vibration.
 12. Microscope according to claim 1, wherein thesingle turn of the loop (27) extends along two branches which aredistinct at a part of the strip (22) linked to the body (20). 13.Microscope according to claim 1, wherein the single turn of the loop(27) extends over the length of the strip (22).
 14. Microscope accordingto claim 1, wherein the electrical-conduction means (26A, 26B) formingthe single turn of the loop (27) have a thickness between 5 nm and 500nm.
 15. Microscope according to claim 1, wherein said single turn of theloop (27) is disposed on at least one surface of the strip (22).